The present invention relates to lightweight clinker with added carbonates of calcium and/or magnesium, for example, dolomite, and the use of such an aggregate for removing phosphorus from, for example, waste water.
Recent years have seen an increase in water purification requirements in the light of growing environmental awareness. Existing treatment plants for removing organic material from water do not, however, remove phosphorus. To remove phosphorus additional purification is necessary in order to bring the amount down to an environmentally acceptable level.
Methods are known involving the use of aluminum or iron salts, lime, or magnesium or calcium oxide. For example, U.S. Pat. Nos. 5,368,741 and 4,402,833 describe the addition of Ca(OH)2 to a settling tank in order to bind and precipitate phosphorus from waste water.
Moreover, it is known from U.S. Pat. No. 5,271,848 to allow polluted water to flow through a layer of bauxite powder or gravel to remove phosphorus. If the gravel is crushed too finely, thus obtaining a large surface and so great reactivity, the hydraulic properties of the mass will be reduced.
Moreover, so-called artificial wetlands are known that are constructed in the form of a bounded permeable earth medium planted with marsh vegetation, through which earth medium the phosphorus-containing waste water, from which most other organic materials have been removed, is filtered. Such plants are described, for example, in a paper read at a symposium of the Norwegian Water Federation (Norsk Vannforening) on Oct. 14, 1991 by Petter D. Jenssen and Trond Maeluum.
A disadvantage of such plants has often been that insufficient attention has been paid to the hydraulic conductivity of the earth medium, i.e., the capacity of the earth medium to conduct water, so that the water is not pressed up and flows over the surface. To achieve sufficiently high phosphorus binding it has in fact been necessary to use sand with a very large specific surface, i.e., sand having very small particles. Sand of this kind has very poor hydraulic conductivity.
It is also known to use light expanded clay aggregates (such as LECA) to improve the hydraulic conductivity of the earth medium. These clay aggregates have also been found to have a certain capacity for removing phosphorus, which is due in part to the fact that during production these balls are often powdered with dolomite in the firing zone for production-technical reasons. The phosphorus binding capacity of these clay aggregates is however not sufficiently great, thus making it is desirable to have a medium having greater capacity for binding phosphorus.
It is an object of the present invention to find a material for use as a filtration medium for purifying water which has a considerably greater capacity for removing phosphorus than the previous expanded clay aggregates, whilst having the same good hydraulic properties as this previously known material.
According to the invention, this is achieved by means of sintered light expanded clay aggregates, characterised in that prior to expansion and firing carbonates of calcium and/or magnesium are added to the clay as flux material.
With the aid of the present sintered light aggregates it has been possible to combine a high phosphorus binding capacity with very good hydraulic conductivity. The sintered light expanded clay aggregates are balls having a large specific internal surface in the form of internal cavities in the configuration of small cells which are interconnected. The reactive carbonates of calcium and magnesium contained in the matrix are spread over this large internal surface and provide a very large phosphorus binding capacity.
The sintered light expanded clay aggregates are ceramic matrices, which affords the material the strength to ensure that it retains its hydraulic conductivity. In this way it is ensured that the water which is to be purified has good contact with the reactive substances in the matrix and that the reactive substances are distributed in the system in an expedient and optimal fashion
The capacity of the present sintered light aggregates to bind phosphorus is also dependent upon a large specific surface, but this large specific surface is obtained in that the internal surface in the light aggregates is also accessible to the water. In the case of previous plants based on sand, it has been necessary to use sand having very small particles in order to obtain a sufficiently great specific surface, which has resulted in an excessively low hydraulic conductivity.
The sintered light aggregates according to the present invention are made following a conventional method for manufacturing light, expanded clay aggregates (Leca), in that marine clay is treated by a process wherein the clay is fed into a rotary kiln where it is first shaped into clay pellets which are fired and finally expanded at a temperature increasing up to about 1200xc2x0 C. in the firing zone. In this way an approximately ball-shaped granulate having a ceramic shell around a porous core is formed. The clay aggregates according to the present invention are prepared in essentially the same way except that prior to granulation and firing carbonates of calcium and/or magnesium, e.g., dolomite, are added to the clay.
Light expanded clay aggregates are ceramic products which upon exiting the kiln are approximately ball-shaped and normally have a diameter within the range of about 0 to 32 mm. The sintered light clay aggregates have an internal structure with a large number of air bubbles in a matrix of fired clay. The outer surface is relatively dense and forms a barrier against free flow of fluids from the surroundings into the ball.
To ensure that the internal surface of the sintered light aggregates is accessible to the water that is to be purified, the balls are preferably cracked prior to use. The word xe2x80x9ccrackingxe2x80x9d is used here as distinct from crushing, since cracking divides up a smaller number of bits and large amounts of excessively small particles are not formed. In this way it is ensured that the water is not prevented from reaching the internal surfaces of the balls"" surface. The cracked light aggregate balls still have such large particle size that the hydraulic conductivity through a tank containing particles of this type is high, whilst the water flowing through encounters a large effective surface which, in addition to the outer surface shell of the balls, also consists of the internal surfaces in the expanded clay aggregates. To ensure this, it is preferred that the cracked clay aggregates have a size distribution of 1 to 10 mm, preferably 1 to 4 mm, and most preferably 2 to 4 mm. A small amount of more finely grained material may also be present, but this material is preferably sieved out as it may prevent through-flow in the filter.
A second object of the invention is to provide an improved method for removing phosphorus from water.
Thus, a method is provided for removing phosphorus from water, such as waste water, where the water is filtered through a filtration medium which retains phosphorus, where as filtration medium sintered light expanded clay aggregates are used that are produced by expansion and firing of a clay-based material and where the finished aggregates have a diameter of up to about 32 mm, and where, prior to expansion and firing, carbonates of calcium and/or magnesium are added to the clay as flux material.